Conventionally proposed boiler systems for combusting a plurality of boilers to generate steam include a boiler system of the so-called proportional control type, for continuously increasing or decreasing a boiler combustion amount to control a steam flow.
For example, Patent Document 1 proposes a method of controlling proportional control boilers that are sectioned into three load zones including a boiler number increasing load zone, an optimum operation load zone, and a boiler number decreasing load zone. According to this method, when any of the boilers is out of the optimum operation load zone and comes into a state of combusting in the boiler number increasing load zone or the boiler number decreasing load zone, the number of the combusted boilers is increased or decreased so that the boilers are combusted in the optimum operation load zone.